The invention relates to the transfer of an analog signal and particularly to a method and equipment for providing galvanic isolation. The invention further relates to a galvanically isolated electronic switch.
Analog signals need to be transferred from one device into another for example in industrial automation systems. Since the control devices are often in a different potential than the devices to be controlled, or because currents flowing in the neutral conductors of the devices may cause errors, a specific technique is needed for transferring the measurement or control signals of the devices to be controlled: if the difference in potential between the controlling device and the device to be controlled is small, then an ordinary differential amplifier is sufficient. When greater differences in potential are concerned, galvanic isolation is used.
The prior art teaches that galvanic isolation can be carried out for example with a converter, capacitor or opto-isolator. The analog signal to be measured is then first converted into a frequency, pulse ratio, light intensity or a digital form, and after the galvanic isolation the analog signal conforming to the original variable is restored. The present prior art galvanic isolators are, however, expensive.
In U.S. Pat. No. 5,361,037 Isolation Amplifier with Capacitive Coupling, the signals of the device to be controlled are isolated from those of the controlling device by means of a linear isolation amplifier circuit. The circuit in question performs the isolation using Switched Capacitor (SC) switching. The circuit is composed of a voltage-to-charge converter and a differential amplifier functioning as a charge indicator. The disadvantages of the circuit in question are, however, the sensitivity of the switching to stray capacitance, the large capacitors needed in the circuit because of the stray capacitance, and high load caused on the input side.
It is an object of the invention to provide a method and an apparatus implementing the method to allow the above mentioned problems to be solved or at least minimized. The object of the invention is achieved with an method and an arrangement characterized by what is stated in the independent claim. The preferred embodiments of the invention are disclosed in the dependent claims.
The circuit of the invention comprises:
a first input terminal and a second input terminal for receiving input voltage;
a first output terminal and a second output terminal for generating output current;
a serial capacitance section for providing galvanic isolation between the input terminals and the output terminals;
first polarity switching means arranged to operationally connect the input terminals to the serial capacitance section by periodically reversing the polarity; and
second polarity switching means arranged to operationally connect the serial capacitance section to the output terminals by periodically reversing the polarity, in synchronism with the first polarity switching means.
The switches of a circuit according to a preferred embodiment of the invention may be provided with small capacitors, which provides such advantage as good tolerance for disturbances, low loading of the measurement point and low power consumption at high frequencies. The circuit of the preferred embodiment of the invention loads the input side only a little, and the circuit of the invention is not sensitive to stray capacitance. In addition, a circuit comprising smaller capacitors requires less silicon space and is therefore more economical than a circuit comprising large capacitors. Moreover, the circuit is simple.
According to another preferred embodiment of the invention, the output current which is in the circuit output terminals and which is proportional to the voltage to be measured, is converted to an output voltage in a second capacitance section. This provides the advantage that no power source is needed for the circuit and it is not necessary to examine in connection with installation whether the analog input, for example, should be isolated or not, because the couplings between the inputs, outputs and the controls of the circuit are galvanically isolated from one another.
According to an alternative embodiment of the invention, the second polarity switching means are followed by active means requiring a power source for generating output voltage.